The present invention relates to a DC-to-DC converter that converts a first input voltage into a first and second output voltage. The converter is very efficient and tightly regulates the output voltages.
As supply voltages required for new silicon chips lower, voltages needed to power up these chips perforates to many different levels in modem computer, communication, networking, and data storage systems. As a result, the power supply modules take more and more internal space of a electronic system. Even in a electronic system using distributed power architecture, the board space taken by the power supplies increases rapidly although the power supply modules+ size and volume have reduced drastically. For many applications, it is more efficient and economical to use multiple output converters in place of two or more power modules.
A traditional way of designing multiple-output converters is to use a multiple winding transformer to provide several outputs. The topologies are often flyback or buck type topologies, either forward or double ended circuits such as bridge or push-pull circuits. FIGS. 1 and 2 shows a multiple-output converter implemented using a flyback and forward converter respectively.
Referring to FIG. 1, a power converter 100 is shown having a single input voltage 102 and a first and second output voltage 104, 106. A switch 108 controls the flow of current to a power transformer 110. The power transformer 110 has a primary winding 112 and a first secondary winding 114 and a second secondary winding 116.
The converter 100 is also known as a xe2x80x9cforward converter.xe2x80x9d It uses a pair of rectifying diodes 118, 120. A certain amount of voltage is lost across the rectifying diodes. Indeed, one of the drawbacks of prior art converters involves the number of components used and the voltage losses across those components. The converter 100 also has a pair of freewheeling diodes 122, 124.
FIG. 2 is a multiple-output converter 200 also having an input voltage 202 and a first and second output voltage 204, 206 . A switch 208 controls the flow of current to a power transformer 210. The power transformer 210 has a primary winding 212 and a first secondary winding 214 and a second secondary winding 216. It also uses a pair of rectifying diodes 218, 220.
Both prior art converters 100, 200 have significant limitations. First, both are known to have poor cross regulation. In other words, the output voltages vary widely, especially when loads are at extreme conditions, i.e., one operating at full load and the other at light load. In the flyback multiple output converter the output cross regulation depends on the parasitics of the power transformer and the voltage drop across the rectifier diodes [218, 220] In the forward multiple output converter, each output needs an inductor to smooth the output ripple. The inductor results in a power loss especially for low voltage, high current applications. The voltage drops across the output inductors and output rectifiers also have adverse effects on the output cross regulation. Further, each output power stage has its own secondary transformer windings, output inductors, and output rectifiers. If each secondary winding is required to deliver the full power, all the components are sized to handle the full power. Therefore under extreme operation conditions, or one output operating at the full load while the other operating at the light load, the lightly loaded part of the circuit is not participating and hence the circuit components are under utilized.
Since the above circuits are relatively easy to implement, they have been used in the industry for years, especially in applications where output voltages are allowed to vary over a certain range such as PC power supplies. Many power supply designers are familiar with the topologies and understand design trade-off. They developed various methods to deal with the cross regulation issue caused by variations of voltage drops across the rectifying devices and output filter inductors such as weighted voltage control and the frequency modulation (FM) coupled with pulse width modulation (PWM) method. These techniques, however, are not without drawbacks. The former control method does not eliminate cross regulation errors but rather redistribute the errors in a controlled manner so designers have the freedom to choose which output is more tightly controlled than the other. The latter can eliminate cross regulation error by introduce another control variable the switching frequency in addition to PWM control. It requires variable frequency control and discontinuous conduction mode operation, both of which are highly undesirable in many applications.
Schlecht disclosed a new circuit in U. S. Pat. No. 5,999,417 that can be used to implement multiple output converters by adding another secondary winding on each transformer as shown in FIG. 3. The converter 300 takes an input voltage 302 and creates a first and second output voltage 304, 306. The front end of the circuit includes a first and second switch 308, 310, a capacitor 312 and an inductor 314. These components are collectively referred to as a xe2x80x9cbuck converterxe2x80x9d 316. The buck converter establishes an intermediate bus voltage, at point 318. Two power transformers 320, 322 are placed in cascade with the buck converter. Each power transformer 320, 322 is connected to switch 324, 326 respectively. Switching on and off the switches create a AC voltage across each transformer""s primary winding. The induced voltage across the secondary windings are rectified by the secondary rectifying devices, thus a DC voltage been established at each output. The advantage of this circuit is that it has all the magnetic components on the primary side and therefore has better cross regulation and higher efficiency compared with the converters of FIG. 1 and FIG. 2. One drawback though is that it has to use two transformers and implementation is relatively complicated.
In light of the problems and drawbacks associated with the prior arts, what is needed is a circuit that can provide tightly regulated outputs with relatively simple structure.
The present invention relates to a new set of dc-to-dc converter topologies. The basic circuit common to each embodiment includes a buck converter to provide an intermediate bus voltage, a double-ended circuit, a single power transformer and at least one output voltage circuit. The new converters are tightly regulated and provide consistent voltage levels even when one output voltage circuit is experiencing a high load. The new converter designs are also highly efficient. They contain fewer active and passive elements than prior art converters and avoid unnecessary duplication of parts.
The front end of the circuit is a buck converter. Buck converters are well known in the art and are used to provide an intermediate bus voltage. A double-ended circuit is then used to couple the buck converter to a power transformer. The double-ended circuit can be either a full bridge circuit, or a push-pull circuit. The full bridge circuit is the preferred embodiment. It includes two pair of switches, each pair operating at a fixed duty cycle, typically an approximately 50% duty cycle. The first pair of switches operate out of phase with a second pair of switches. Further, either pair of switches comprises a first switch on a first terminal of the primary winding a second switch on a second terminal of the primary winding.
The power transformer has a primary and a secondary winding. In one embodiment, a single secondary winding is used. In another embodiment, two secondary windings are used. An output voltage circuit is coupled to each. In another embodiment, a single secondary winding is used with two output voltage circuit coupled to different terminals on the secondary winding. The output voltage circuits use rectifying devices, either rectifying diodes or rectifying switches. Additional drive windings can be used between the rectification switches and the secondary windings. Further, additional bias voltage circuits can be coupled to the power transformer.